In vehicles, use of thinner panels has been required for saving energy. This results in reduced strength, which causes automobiles insufficient surface tensional rigidity and insufficient solid appearance.
In order to solve the problems, there were proposed various processes for producing a vehicle outer panel structure having lightweightness. For example in the laid open of Japanese Patent Application No 1988-272515, there was proposed a process which comprises attaching to the backside of a panel sheet material having a three layer structure consisting a first layer which is a foamable composition comprising as essential component, a vinyl chloride resin, a plasticizer, a high temperature decomposition type expansion agent, an epoxy resin and a heat activation type hardener for epoxy resin, a second layer which is a cloth, and a third layer which is a thin metal film, and then heating the resulting panel to allow the sheet material to give resin to expand and hardening.
In this process, however, the foamable composition contains a vinyl chloride resin as a thermoplastic resin, and therefore has poor adhesion to oily surface and, when heated to give rise to expand and hardening, gives a sheet of poor heat resistance.
Hence, the process is applicable to vehicle outer panels after electro deposition but inapplicable to vehicle outer panels before electro deposition.
In the automobile industry, attaching of reinforcing sheet material to vehicle outer panel before electro deposition is strongly desired for higher productivity or for use of such a reinforcing sheet material to as many applications as possible.
Especially, the reinforcing material for the quarter panel part and the door fender panel of vehicles uses a stainless steel or a fiber grass as the restraint layer, and an unexpandable pad is adhered to the backside of the restraint layer with the thickness of 1.5.about.2.5 mm. And finally a releasing paper is covered on the unexpandable type pad to protect the unexpandable pad.
That is, since the quarter panel and the door fender panel of the vehicle have the curved shape, the expandable pad is used as the reinforcing sheet material of the curved portion of vehicle. Then the reinforcing sheet material for the quarter panel and the door fender panel is separated partially after electro deposition. And water or a foreign substance is inserted into the gap between the panel and the reinforcing sheet, which causes the panel of the vehicle to get rust.
On the other hand, in order to solve the problem the glass wool is used as the restraint layer, the glass wool also cannot give a sufficient solution.